Three-Parent Embryos: Scientific Concerns

Last week, I was interviewed about the FDA hearings that considered human trials for the three-parent embryo.[1] We spent much of the interview talking about the ethical concerns, but I think it is important to first address the science that’s involved. Part of helping us to think bioethically includes a responsibility to understand basic science.

So, what is the science behind the three-parent embryo?

It’s based on the fact that some serious diseases are transmitted through maternal mitochondrial DNA. Most of the DNA in a cell is found in the nucleus. But, there is also DNA in the mitochondria, the powerhouses that float around in the cytoplasm surrounding the nucleus in the egg. The DNA can carry diseases that can cause strokes, diabetes, epilepsy, and a particularly horrible one that destroys brain tissue.

At this point there are two techniques that could be used to create the three-parent embryo: In one, the nucleus is extracted from an egg belonging to the intended mother, and is placed in a donor’s healthy egg that has been enucleated—the nucleus removed. It is then fertilized with the father’s sperm. This is the technique that was used to produce five healthy monkeys several months back. In the other technique, a nucleus from a fertilized egg—an embryo—is placed in the enucleated donor egg.

Here are some scientific concerns. One is that there simply isn’t enough data from animal trials to predict safety in human beings. The tests on the monkeys were not conducted under “real world” conditions, where the pregnant woman’s diet is not carefully controlled. (Think “ice cream and pickles.”) The recent controversy over the ineffectiveness of Plan B for obese women illustrates that clinical trials often can be out of sync with the realities of actual patients.

Second, one speaker at the hearings noted that even minor variations in the complex cellular environment could affect embryonic development. The mitochondrial DNA from the donated egg might not by compatible with the nuclear DNA.

Third, we have no idea if the harm we want to prevent is even worth the unknown risks. The three-parent embryo is an example of irreversible genetic manipulation, changing not only the first embryo, but every descendant of that person. We know little about the impact on future children, who are unable to consent to being experimental subjects. Researchers should exercise scientific restraint, particularly when there is potential for an unknown impact on vulnerable populations. While we want to be careful about always raising the specter of the “slippery slope,” science is irresponsible if it ignores the high risks of prematurely experimenting on our children.

My list of ethical and social concerns fills several pages. For now, it’s important to understand that good science—responsible science—dictates that the FDA should not approve experimenting with three-parent embryos.